Saturn's moon Enceladus may have rolled over to put a hot spot at the pole

Enceladus, a small icy moon of Saturn, may have dramatically reoriented relative to its axis of rotation, rolling over to put an area of low density at the moon's south pole. According to a new study, this reorientation process could explain the polar location of a region where NASA's Cassini spacecraft recently observed icy jets and plumes indicating active geysers of water vapor spewing from the moon's surface.

"When we saw the Cassini results, we were surprised that this hot spot was located at the pole. So we set out to explain how it could end up at the pole if it didn't start there," said Francis Nimmo, assistant professor of Earth sciences at the University of California, Santa Cruz.

Coauthor Robert Pappalardo worked on the study while at the University of Colorado and is now at NASA's Jet Propulsion Laboratory in Pasadena. Nimmo and Pappalardo have proposed a reorientation process driven by an upwelling of warm, low-density material inside Enceladus. A similar reorientation process may also have operated on other small moons in the solar system, such as Uranus's moon Miranda, they said. The researchers described their findings in a paper published in the June 1 issue of the journal Nature.

Nimmo and Pappalardo calculated the effects of a low-density blob beneath the surface of Enceladus and showed that this could indeed cause the moon to roll over and put the low-density blob at the pole. Rotating bodies, including planets and moons, are most stable if most of their mass is close to the equator. Therefore, any redistribution of mass within the object can cause instability with respect to the axis of rotation. The resulting reorientation will tend to position excess mass at the equator and, conversely, areas of low density at the poles, Nimmo said.

"The whole body rolls over, while the spin axis stays fixed," he said.

An upwelling of warm, low-density material could also help to explain the high heat flux and striking surface features observed at Enceladus's south pole. These features include not only geysers, but also a "tiger stripe" pattern suggesting fault lines caused by tectonic stress.

Image: This graphic illustrates the interior of Saturn's moon Enceladus. It shows warm, low-density material rising to the surface from within, in its icy shell (yellow) and/or its rocky core (red). A NASA-funded study says Enceladus might have rolled or rotated itself to place this area of low density at the south pole. This finding is in the June 1, 2006, issue of the journal Nature. This graphic uses parts of visible-light images taken by the Cassini spacecraft camera that have been modified for the purpose of showing the interior structure. Cassini's cameras captured a giant plume blasting icy particles into space. Full-Res: PIA08500

"The whole area is hotter than the rest of the moon, and the stripes are hotter than the surrounding surface, suggesting that there is a concentration of warm material below the surface," Nimmo said.

Internal heating of Enceladus probably results from its eccentric orbit around Saturn. The gravitational pull Enceladus feels from Saturn changes in the course of its orbit, and the resulting tidal forces generate heat inside the moon.

"Enceladus gets squeezed and stretched by tidal forces, and that mechanical energy is transformed into heat energy in the interior," Nimmo said.

The upwelling blob (called a "diapir") may be within either the icy shell or the underlying rocky core of Enceladus, he said. In either case, as the material heats up it expands and becomes less dense, then rises toward the surface.

The reorientation scenario leads to testable predictions, Nimmo said. For example, the leading hemisphere of a moon as it travels through space should have more impact craters than the trailing hemisphere. But if the moon rolls over, the pattern of impact craters will also be reoriented. A low-density mass may also produce an observable anomaly in the moon's gravitational field.

Additional observations of Enceladus are planned for the Cassini mission and should enable the researchers to test these predictions, Nimmo said.